With the continuing evolvement of the electronics industry, new techniques are continually needed to allow not only incremental progress, but also (albeit typically less often) major technological leaps that become the impetus for another round of incremental progress. For example, in the manufacturing of displays, for example, flat-panel displays such as video, television and computer monitors, among others, substrate sizes have been increasing incrementally over the approximately seven generations of flat panel display technology. The initial substrate size of the first generation of flat panel displays was roughly 320 mm×400 mm. This has increased to about 1800 mm×2100 mm in the current (seventh) generation of flat panel displays. However, these ever-increasing substrate sizes create significant manufacturing and engineering challenges with regard to their use, handling and transportation. In addition, the upfront capital investment in infrastructure required to process these large sheets of glass for each subsequent generation of fabrication has ballooned to upwards of $2 billion per fabrication facility.
Furthermore, future trends in the display/electronics industry suggest that future display and electronic products will be made on flexible/conformal substrates. This transition is seen as inevitable to service the ever present need and desire to reduce the size, weight and cost of devices we use without sacrificing performance. A wide gamut of devices, such as displays, electronics and sensors, to name a few, would benefit from methodologies that would result in the mass production of ruggedized, light-weight, portable, small-form-factor, less power hungry and lower-cost devices. Moreover, new and novel markets and opportunities could be addressed and opened-up if these devices could be made flexible and/or conformal.
To counter the ever-growing substrate-size dilemma and to service future flexible display needs, attempts have been, and are being, made to develop manufacturing processes that would allow for roll-to-roll, or reel-to-reel (also call “web coaters”), technologies. These technologies would allow flexible substrates, such as polymer/plastic foils and metal foils, to be substituted for rigid glass substrates. However, attempts so far have had limited success, primarily due to the complexity of manufacturing active electronic devices, such as field-effect transistors (FETs) that form the basis of most electronic circuitry (note that thin-film transistors (TFTs) are typically in the form of FETs). Typical manufacturing of such devices requires multiple coatings deposited at high temperatures and interspaced with multiple photolithographic patterning steps.
It is commonly known that polymers/plastics, if used as substrates, severely limit the maximum temperature that may be used during device manufacturing. In addition, to prevent undue out-gassing and contamination of equipment and devices during coating deposition, these substrates need to undergo a complex and time-consuming pre-bake thermal cycling step. This step also serves to expel moisture and humidity from the native polymer substrate, thereby stabilizing the coefficient of thermal expansion of the substrate, which is helpful in the photolithographic patterning and pattern alignment steps. Metal foils are more resilient and tend to be immune from this temperature limit imposed by polymer/plastic substrates. However, to date, TFT devices made on metal foils have exhibited low electronic performance due to contamination effects and “unknowns” attributed to high surface-roughness of starting metal substrates.
In addition, the use of flexible substrates has placed heavy demands on engineering new ways and equipment to address dimensional stability of substrates during lithography, mechanics for handling substrate curvature, registration accuracy and consistency of placement of TFTs and electrodes. Furthermore, flexible polymer/plastic substrates have had issues with moisture absorption and resistance to solvents and other chemicals. One of the more significant of these technical challenges that has slowed, and even stymied, attempts at roll-to-roll manufacturing of electronic devices on either polymer/plastic or metal foils is the issue with photolithographic registration and alignment due to the number of coatings and photomasking steps involved in the manufacturing of traditional TFTs.
Pick-and-place techniques wherein complete and/or partial circuits are manufactured in a silicon (semiconductor) wafer and then transferred onto a separate substrate and interconnected to form electronic articles have been known in the semiconductor industry for some time. A variant of the pick and place method is the “fluidic suspension assembly,” or FSA, process, a technique patented by Alien Technology, wherein the manufactured “circuits” are floated into specific locations using a fluidic media and surface chemistry.
Yet another technique, pioneered by Dr. John Rogers and others at the University of Illinois, is a so-called “top down” micro-technology approach to creating high performance active flexible electronic circuits. In short, this group of researchers has created free-standing micro-and nano-scale objects of single crystal silicon (and other semiconductors) from silicon-on-insulator wafers by lithographic patterning of resist, subsequently etching the exposed top silicon, and removing the underlying SiO2 to lift-off the remaining silicon. The free-standing silicon objects so obtained are then deposited and patterned, by dry transfer printing or solution casting, onto mechanically pliant substrates (like plastic) to yield mechanically flexible thin film transistors. They have coined these objects as “microstructured silicon.” Modified versions of the same basic technique are being pursued by Dr. Max Lagally at the University of Wisconsin (SiGe and Strained SiGe crystals), Triton Systems and Si2Technologies, among others.